ARM: 7643/1: sched: correct update_sched_clock()
authorJoonsoo Kim <js1304@gmail.com>
Sat, 9 Feb 2013 04:52:45 +0000 (05:52 +0100)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Thu, 28 Feb 2013 14:59:04 +0000 (06:59 -0800)
commit27c303244ef8049be6fd8088230c7f5e02a522b3
treeccb85f0306747f1b44f919fc798d36bc70d2cd1e
parentcae53e2a9a03e53152fdd331067923f7d65fc72e
ARM: 7643/1: sched: correct update_sched_clock()

commit 7c4e9ced424be4d36df6a3e3825763e97ee97607 upstream.

If we want load epoch_cyc and epoch_ns atomically,
we should update epoch_cyc_copy first of all.
This notify reader that updating is in progress.

If we update epoch_cyc first like as current implementation,
there is subtle error case.
Look at the below example.

<Initial Condition>
cyc = 9
ns = 900
cyc_copy = 9

== CASE 1 ==
<CPU A = reader>           <CPU B = updater>
                           write cyc = 10
read cyc = 10
read ns = 900
                           write ns = 1000
                           write cyc_copy = 10
read cyc_copy = 10

output = (10, 900)

== CASE 2 ==
<CPU A = reader>           <CPU B = updater>
read cyc = 9
                           write cyc = 10
                           write ns = 1000
read ns = 1000
read cyc_copy = 9
                           write cyc_copy = 10
output = (9, 1000)

If atomic read is ensured, output should be (9, 900) or (10, 1000).
But, output in example case are not.

So, change updating sequence in order to correct this problem.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
arch/arm/kernel/sched_clock.c